(1) Field of the Invention
This invention relates to an improved process for the delignification of lignocellulosic material, such as wood, with an alkaline pulping liquor of the kraft, or sulfate, alkaline pulping process. More particularly, the invention relates to lignocellulosic material delignification with kraft pulping liquors containing anthraquinone as a pulping additive. Specifically, the invention relates to a method of solubilizing anthraquinone for introduction into the pulping process.
(2) Description of the Prior Art
The processing of lignocellulosic material to produce cellulose suitable for the manufacture of paper products involves the removal of lignin and other non-cellulosic components such as gums. Reagents that attack lignin without affecting appreciably the cellulose component are preferred for this purpose. In the sulfate or kraft process lignocellulosic material is cooked with a mixture of sodium hydroxide and sodium sulfide. This alkaline solution is called "white liquor" when freshly prepared and is referred to as "black liquor" when recovered from the pulping process. In the soda process the cooking is carried out with sodium hydroxide alone.
Recently, U.S. Pat. No. 4,012,280 to H. H. Holton disclosed that certain cyclic keto compounds, in particular anthraquinone and its derivatives, when employed as an additive in the kraft and soda pulping processes act to accelerate the pulping action and increase yield.
Unfortunately, anthraquinones are not generally soluble in the alkaline pulping liquors prior to cooking and this could cause problems in using them commercially. For this very reason, T. J. Fullerton, in Appita 32(2): 117-18 (1978), suggests soda pulping with anthrahydroquinones, which are soluble in alkaline solution.
In a paper by Holton and Chapman (Tappi 60(11): 121-5 (1977)), 0.05% anthraquinone was used for kraft pulping southern pine chips to produce linerboard grade pulp. The method of introducing the anthraquinone into the digesters in these mill trials was to add the anthraquinone "continuously at the bottom of the chip conveyor belt, on top of the chips."
Anthraquinone is not considered to be a particularly toxic chemical. It has a slight allergic effect on sensitive people. However, because it is usually handled as a very fine powder, it is difficult to prevent significant amounts from becoming suspended in the air. This can even lead to an explosion hazard. Because of this, protective clothing, including rubber gloves and a dust mask, is recommended when handling large amounts of anthraquinone powder.
The ability to solubilize or stably disperse anthraquinone in kraft pulping liquors so that it may be added to the chips as a homogeneous solution or dispersion would provide important advantages in industrial applications.
Surprisingly, it has been discovered, after unsuccessful attempts to achieve a stable dispersion of anthraquinone in white liquor, that the addition of black liquor to the anthraquinone-white liquor mixture causes the anthraquinone to disappear into solution or into a finely dispersed colloidal system.